Exposure apparatus are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that positions a reticle, an optical assembly, a wafer stage assembly that positions a semiconductor wafer, a measurement system, and a control system. The measurement system monitors the position of the reticle and the wafer, and the control system controls the stage assemblies to adjust the positions of the reticle and the wafer. Precise positioning is required for transferring small features typically required. Systems have been developed for determination and correction of positioning errors based on interference of measurement and reference beams such as those disclosed in Goodwin and Smith, U.S. Patent Application Publication 2013/0048842, and Liu, U.S. Patent Application Publication 2013/0128255, both of which are incorporated by reference. However, further improvements are needed, particularly to reduce cost and complexity.